(a) Field of the Invention
The present invention relates to hearing aids, and more particularly, to a hearing aid for delivering acoustic signals of a substantially constant average signal level to persons with moderate to severe hearing loss.
Hearing aids are apparatuses, normally electronic, for amplifying sound and supplying it to the ear of a person whose hearing is impaired. A microphone or other transducer converts sounds, particularly speech, or other signals into an electrical signal which is then amplified and supplied to an earphone inserted in the outer ear of the patient.
It is very common for a hearing impaired person to find that a hearing aid helps him to understand speech from one person in a quiet room but is useless if others are talking at the same time. The reason for this is that a patient who has reduced sensitivity of hearing, perhaps due to sensori neural deafness, usually has other deficiencies as well which are not normally measured.
The parameters important for speech understanding, particularly in noise, include:
Sensitivity: if the average hearing loss at 500, 1000 and 2000 Hz is less than 35 dB a patient can manage well unaided in quiet situations. If the loss is greater, he will benefit from amplification of the weaker speech components;
Recruitment: this is common, particularly with sensori neural deafness, which accounts for 80% of all deafness. The threshold of hearing is elevated. Below it the patient hears nothing. Above it the patient hears with the intensity of a normal ear so that sounds are either inaudible or loud;
Frequency discrimination: a healthy ear can detect a 1% change in frequency of a tone. A bad ear may only detect a 20% change, or even hear noise rather than a tone;
Frequency selectivity: the peripheral auditory system analyzes the incoming complex sounds of speech into their component frequencies by what are called auditory filters. If the ability to do this is impaired, speech recognition, particularly in noise, becomes very difficult. Understanding speech depends on comparing moment to moment changes in the spectrum of the speech sound;
Temporal discrimination: normal ears can perceive gaps in speech as small as 3 mS. Impaired ears may not perceive gaps of 10 mS. For speech recognition small gaps must be recognised. Noise makes matters worse by filling in the gaps; and
Temporal masking: if a weak sound follows after a loud sound it may not be heard. This effect is often worse with impaired ears and causes weak consonants after loud vowels to be lost. Noise makes matters worse by masking weak consonants even further.
Not all these difficulties may be present in any particular patient, but a patient with an audiogram showing reduced sensitivity often has deficiences in some of these parameters as well.
A peculiarity of speech is that the vowels are loud and of low frequency and consonants are weak and of high frequency. Another peculiarity of speech is that most of the information is in the consonants.
FIG. 1 of the accompanying drawings, which is a graph of sound level against frequency, shows the threshold of hearing (THL) and the threshold of discomfort of a sensori-neural deaf patient. The threshold of discomfort is the same as for normal hearing whereas the threshold of hearing is raised compared with a normal THL. The temporal and frequency parameters which are needed to perceive speech tend to be better above an area which is labelled area of distortion, so that speech should be presented to the patient in the area above the area of distortion.
FIG. 2 of the accompanying drawings, which is a graph of sound level against frequency, shows the long term spectral distribution of ordinary quiet speech as the area between chain dotted lines to ensure that all components are heard by the patient, the speech envelope should be processed so that it fits between the broken lines. This means that high frequencies should be amplified more than low frequencies and that the dynamic range, particularly at high frequencies, should be reduced. Low frequencies must be prevented from becoming too loud or they will mask high frequencies.
(b) Description of the Prior Art
One previous but unsuccessful way of trying to achieve this is to provide a tone control to make the amplification greater at high frequencies, and then use automatic volume control (AVC) to reduce the dynamic range. This does not work because all the energy in speech is in the low pitched vowels and most of the information is in the weak high pitched consonants. AVC causes amplifier gain to drop when a loud vowel occurs so that a following weak consonant is suppressed and is not heard.
FIG. 3 of the accompanying drawings is a block circuit diagram of another known hearing aid of the type disclosed in EP 0077688. Signals from a microphone A1 are supplied to another amplifier A2. The amplifier A2 has conventional AVC which only operates at high speech levels above 70 dB SPL and is used to prevent very loud speech from overloading the system. The attack and release time constants are 2 milliseconds (mS) and 300 mS, respectively. The speech signal is then split into an upper frequency band above 1500 Hz and a lower band below 1500 Hz by high pass and low pass filters A3 and A4, respectively. The degree of compression needed by the patient is set in amplifier A5 for high frequencies to suit his dynamic range. Low frequency vowels do not go through this channel. The amplifier A5 limits the dynamic range of the signals with attack and release time constants of 2 mS and 10 mS, respectively. Similarly, the appropriate compression is set in a limiting amplifier A6 for low frequencies having attack and release time constants of 2 mS and 30 mS, respectively. More compression than necessary is used here so that loud sounds do not mask high frequencies. A mixer A7 combines the outputs of the amplifiers A5 and A6 and supplied the combined signal to a power amplifier A8 whose gain is adjusted to suit the threshold of discomfort of the patient. A receiver or earphone A9 delivers speech to the patient's ear.
In such a hearing aid, the amplifier A2 starts compressing or limiting at a fairly high threshold. Also, a relatively simple AVC system is used with attack and release time constants which are a compromise between conflicting requirements. Namely, first of all the input AGC amplifier should present all speech signals to the rest of the hearing aid at the same average level. Otherwise, the high frequency AGC channel will recover between speech peaks when high intensity speech is presented to the microphone and the hearing aid will sound noisy. Secondly the input AVC amplifier should extract the true average value of the incoming speech. The hearing aid shown in FIG. 3 uses input AVC with an attack time of 2 mS, so that the patient is protected against loud noises like a bang of a door, and a 300 mS release time to 90% of full sensitivity. To extract a real average, the release time should be much longer, for instance about 5 S, but then the bang of a door would disable the aid for an obtrusive time.